1. Field of the Invention
The present invention relates generally to optically-coupled chemical sensing devices and to processes for detecting the presence of certain classes of chemical compounds. More particularly the invention concerns an apparatus for use in sensing pollutants in water such as wastewater.
2. Discussion of the Invention
Introduction
Electroluminescence occurs in semiconductor materials that are capable of emitting visible or near visible radiation when an electrical current passes through the semiconductor. Photoluminescence can also occur in these materials. If external light is used to excite the semiconductor, a characteristic wavelength of light is emitted. These characteristic wavelengths vary amongst different photoluminescent semiconductors and can be varied in a single semiconductor by doping or changing the composition of the material.
Amongst the various studies on the luminescence of photostimulated or electroluminescent materials is "Luminescent Photoelectrochemical Cells", Streckert, H. H., Tong, J. and Ellis. A. B., J. Am Chem. Soc. Vol 104, No. 2, 1982, pp 581-588. It is noted therein that the intensity of light emitted by electroluminescence and photoluminescence varies directly with the applied voltage. The efficiency of charge transfer and good electrical contact at the surface is also noted as important in the efficiency of the process.
U.S. Pat. No. 4,780,643 discloses semiconductor electrodes having multicolor luminescence. These semiconductors comprise solid-state solutions of three elements that vary in a vertically anisotropic manner. The preferred solid-state solutions are of cadmium, sulfur and selenium.
U.S. Pat. No. 4,211,586 discloses a method of forming a multicolor light-emitting array of diodes. The diodes are formed by differentially etching a graded n-type semiconductor and diffusing a p-type dopant into the surface of the n-type semiconductor to form a p-n junction diode.
U.S. Pat. No. 4,645,932 discloses an apparatus for detecting the presence of certain chemical compounds comprising a photoluminescent semiconductor having a metal coating on a radiation-emitting surface of the semiconductor, a source of actinic radiation that can impinge on the radiation-emitting surface of the semiconductor, and a means for detecting changes in the characteristics of radiation emitted from said radiation-emitting surface. The absorption of hydrogen into the metal layer is suggested as varying the height of the Schottky barrier of the diode and causing a change in spectral characteristics of the radiation emitted.
U.S. Pat. No. 4,710,476 discloses an optically-coupled sensing apparatus of a photoluminescent semiconductor, a source of actinic radiation that can impinge on a radiation-emitting surface of the semiconductor, and a means for detecting changes in the characteristics of the radiation emitted from the radiation-emitting surface. The radiation-emitting surface has reacted thereon a compound capable of undergoing oxidation and/or reduction, the redox product of which reacted compound has a vertical charge distribution therein with respect to the radiation-emitting surface.
U.S. Pat. No. 4,752,588 discloses an optically-coupled sensing apparatus of a photoluminescent semiconductor, a source of actinic radiation that can impinge on a radiation-emitting surface of the semiconductor, and a means for detecting changes in the characteristics of the radiation emitted from the radiation-emitting surface. The radiation-emitting surface has reacted thereon a compound is capable of bonding to certain gases, undergoing oxidative addition and/or reductive elimination reactions, resulting in a change in the vertical charge distribution with respect to the radiation-emitting surface.